The present invention relates to electromagnetic deflection yokes for use with television picture tubes.
Both color and monochrome picture tubes employ deflection yokes positioned to horizontally and vertically deflect the electron beam or beams over phosphor covered viewing screens. There are two common types of yoke winding configurations--saddle and toroidal. Frequently, different types of winding configurations are employed for the separate functions of horizontal and vertical deflection. Such an arrangement is commonly referred to as a hybrid yoke. The present invention relates specifically to hybrid yokes having saddle wound horizontal deflection coils.
Recently picture tubes having horizontal "in-line" electron guns, distinguished from the more traditional delta gun arrangement, and viewing screens with vertical phosphor strips have become popular, primarily because of the greater ease of obtaining convergence. In fact, hybrid yokes are known in the prior art which fully achieve beam convergence without the necessity of providing external correction. Furthermore, in addition to self-convergence, hybrid yokes are known in the prior art having north/south pincushion correction designed into the yoke's horizontal saddle coil.
The design parameters for achieving a self-converging, north/south pincushion corrected deflection yoke, which generally become more critical as the yoke deflection angle is increased, are generally well known in the art. The design parameters defining a deflection coil are normally represented in terms of the Fourier coefficients of the spatial distribution of wire turns density comprising the coil. The predominant coefficients are the first, third and fifth spatial harmonics of the expanded Fourier series. For convenience in dealing with the physical structure of a coil, these coefficients are frequently translated into corresponding moment angles. Thus, the coefficents corresponding to the third and fifth harmonics of the winding distribution essentially constitute, respectively, the first and second moment winding distribution parameters of the coil. In terms of the foregoing parameters, it is known that in order to achieve north/south pincushion correction it is necessary to wind the horizontal saddle coil having a winding distribution exhibiting a low first moment (also frequently referred to as the "center of gravity") or, stated otherwise, a large third harmonic content at the front of the coil, i.e. at its end adjacent the flared portion of the picture tube. Self-convergence is obtained by configuring the horizontal coil for converging the beams along the horizontal axis of the tube and by properly matching the coil configuration with a self-converging vertical coil (a vertical coil converging the electron beams on the vertical axis of the tube) to obtain convergence throughout the raster. The first requirement, convergence along the horizontal axis, is achieved by designing the horizontal saddle coil so that its winding distribution is characterized by a particular average first moment computed along the axial length of the yoke (corresponding to particular average third harmonic content) depending upon the specific geometry of the yoke-tube system. The second requirement, matching the horizontal saddle coil with a self-converging vertical coil to obtain convergence throughout the raster, is achieved by configuring the horizontal coil for exhibiting a winding distribution having a relatively large average second moment (frequently referred to as spread) or, correspondingly, a large average fifth harmonic content. Due to physical winding constraints, the large second moment parameter is typically incorporated into the back end of the horizontal coil if pincushion correction is designed into the coil's front end (i.e., a low first moment parameter). However, a saddle coil designed in this manner is relatively difficult to wind in that the initial turns laid down must extend between a point at the rear end of the coil having a large angular displacement from the horizontal plane passing through the central axis of the yoke to a point at the front of the coil having a relatively small angular displacement from the foregoing plane.
Other prior art attempts to achieve the foregoing design parameters, individually or in combination, have taken various forms. For example, in U.S. Pat. No. 3,849,749 to Kadota a yoke is disclosed having three axially spaced deflection coils, each coil being configured for producing an appropriate field such that the combined field suitably corrects pincushion and convergence distortions. In another embodiment, the Kadota patent illustrates the use of yoke half-sections tilted towards the tube screen for achieving the required fields. Other known approaches involve the use of magnetically permeable shims attached to the coils to appropriately influence the resulting magnetic fields as well as "denting" the coil in suitable locations, normally at the back end of the coil, thereby affecting the resulting center of gravity and/or spread.
However, in addition to designing a hybrid yoke exhibiting the above design parameters, it would also be highly desirable to provide a yoke wherein the horizontal saddle coil is rapidly and repeatably producible, for example, on a conventional tape controlled winding machine. Of course, the use of shims and dented coil sections would be unacceptable in such an application. In this regard, U.S. Pat. No. 3,855,694 to Van Der Heijde discloses the method of winding horizontal saddle coils in which the turns are distributed over a number of sections separated by void spaces or apertures by inserting one or more pins into the winding space during winding whenever the number of turns required for one section is obtained. Although this winding process is in and of itself attractive from a yoke manufacturability or windability viewpoint, the resulting coil does not exhibit the necessary magnetic characteristics for achieving north/south pincushion correction and self-convergence. In fact, the Van Der Heijde patent addresses the design problem of providing a yoke for a delta gun system, a solution for which would not be directly relatable to an in-line gun system.